New human ancestor was Lucy’s cousin and neighbor

The famous human relative known as “Lucy” has reigned alone as queen of an important time and place in human evolution: Ethiopia about 3.2 million years ago, roughly the time when the first stone tools appear in East Africa. Now, scientists working near where Lucy was found claim that freshly discovered jawbones and teeth belong to a previously unknown species of human relative. If correct, the finding could confirm other possible evidence that Lucy did not walk alone, and expand the pool of potential ancestors for our own genus, Homo. But some skeptics argue that the new fossils might be variant individuals of Lucy’s own species.

Since Lucy was discovered in Ethiopia’s Afar region in 1974, researchers have uncovered many more fossils assigned to her species, Australopithecus afarensis, and dated to between 3.7 million and 3.0 million years ago. This is a crucial window for human evolution, with the earliest claimed stone tools, found in neighboring Kenya, dating to 3.3 million years ago. And the earliest known member of the Homo genus is now thought to date to about 2.8 million years ago. This proximity in time, and anatomical features that appear to foreshadow those of early humans, have made Lucy’s species the main contender as a direct ancestor of Homo. But the picture has become murkier in recent years, as research teams working in Kenya and farther away in Chad have identified fossils contemporaneous with Lucy that they propose belong to two other species—also candidate human ancestors. But other researchers dispute whether these fossils are different enough to represent a new species.

In this week’s issue of Nature, a team led by Yohannes Haile-Selassie, a paleoanthropologist at the Cleveland Museum of Natural History in Ohio, reports uncovering parts of two upper jaws and two lower jaws, plus some associated teeth, from a new possible species in the Afar region of Ethiopia, just 35 kilometers north of where Lucy was found. The purported new species, which the team calls Australopithecus deyiremeda (in the Afar language, deyi means “close” and remeda means “relative”), was found in sediments dated between 3.5 million and 3.3 million years ago, meaning that it overlaps both in time and place with Lucy's and her kin. Haile-Selassie and his colleagues argue that the fossils differ from those of Lucy’s species in several ways, having smaller teeth, more forward-facing cheekbones, a more robust lower jaw, and thicker outer enamel on some teeth.

How could two different but closely related species of Australopithecus live in the same time and the same place? “If enamel thickness and jaw robusticity are any indications of dietary adaptation,” Haile-Selassie says, then A. deyiremeda “was probably adapted to harder, tougher, and more abrasive dietary resources,” such as tough plants and grasses, than Lucy's species. Although the two claimed species clearly overlapped in time, Haile-Selassie says, they may not have arisen simultaneously. The fossils found so far suggest that A. deyiremeda came on the scene after Lucy’s species and could even have branched off from it.

Carol Ward, an anthropologist at the University of Missouri, Columbia, says that the claim for a new species is reasonable based on the limited evidence. The fossils “do fall outside of the range of variation of any species described so far,” Ward says, although she thinks more specimens will be needed to make a truly valid comparison. “There is definitely diversity” in the human relatives of this time, meaning that Lucy probably was not alone, Ward says. Instead, she was likely accompanied by different australopithecines occupying a variety of ecological niches. “This flexibility may have been a key reason for the success of what now appears to be an early radiation” of human relatives, out of which Homo ultimately arose, Ward adds.

But William Kimbel, a paleoanthropologist and Lucy expert at Arizona State University, Tempe, says when comparing Lucy and the new species, “the distinctions in my view are pretty subtle.” Anthropologist Tim White of the University of California, Berkeley, is even more skeptical. “The slight anatomical differences noted in this case fall short of demonstrating biological species diversity,” he says. “Lucy’s species just got a few more new fossils.”